In the past, for example, there has been a technology described in Patent Literature 1 as a technology concerning a digital multiplexing apparatus and a digital demultiplexing apparatus adapted to multiple rates. The operations of a digital signal demultiplexing apparatus and a digital signal multiplexing apparatus described in Patent Literature 1 are explained below. This digital signal demultiplexing apparatus includes an A/D (Analog to Digital) conversion unit that converts an analog signal into a digital signal, a plurality of reception half-band filters that allow frequencies corresponding to respective bands to pass, down-sample a sampling rate to a half of an input data rate, and output the frequencies, a reception selector unit, a frequency reverse offset unit, a reception-waveform-shaping filter unit, and a demodulation processing unit.
The digital signal multiplexing apparatus described in Patent Literature 1 includes a modulation processing unit, a digital multiplexing unit, a transmission-waveform-shaping filter unit, a transmission selector unit, a frequency offset unit, a plurality of transmission half-band filters that up-sample a sampling rate to a double of an input data rate, allow frequencies corresponding to respective bands to pass, and output the frequencies, an adder, and a D/A (Digital to Analog) conversion unit.
Each of the reception half-band filters includes a band-pass filter and a down-sampler. The band-pass filter has frequency characteristics suitable for a band to which the band-pass filter corresponds. Each of the transmission half-band filters includes an up-sampler and a band-pass filter. The configuration of this band-pass filter is the same as that of the reception half-band filter.
In a signal spectrum after passing the band-pass filter of the reception half-band filter, an input signal in a pass-band directly passes and a signal in a stop-band is removed. A signal in a transition region located between the pass-band and the stop-band is not completely removed and remains as a triangular spectrum.
The down-sample in the reception half-band filter down-samples a sampling frequency fsamp to a half with respect to a signal after passing the band-pass filter. According to this down-sampling processing, a signal component present in a band of 0.5 fsamp to 1.0 fsamp [Hz] before the down-sampling is superimposed in a frequency axis direction on a signal component present in a band of 0 to 0.5 fsamp by aliasing. Therefore, a signal component in a pass-band overlaps a signal component in a stop-band. However, because the signal component in the stop-band is removed by the band-pass filter, it is possible to realize down-sampling without causing deterioration in an SN (Signal to Noise) ratio of the signal component in the pass-band.
The digital demultiplexing apparatus in the past repeats, for example, the band-pass filtering processing and the down-sample processing set to frequency characteristics corresponding to any one of bands #0 to #3 until a frequency is down-sampled to a desired sampling frequency and a desired signal band.
The digital demultiplexing apparatus selects data, which is designated by a channel control signal from a system side, from complex baseband data (a signal after filtering) output from each of the reception half-band filters and outputs the data. For example, when four outputs are selected, the frequency reverse offset unit, the reception-waveform-shaping filter, and the demodulation processing unit in the post-stage apply signal processing to these four data.
On the other hand, in a complex baseband signal down-sampled to a desired sampling frequency fAD/(2n) [Hz] by the processing by each of the reception half-band filters, a center frequency is not zero and moves to fAD/2(n+2) [Hz]. In general, the demodulation processing unit at the post-stage performs detection processing with a center frequency of a baseband signal set to zero. Therefore, the frequency reverse offset unit frequency-converts the center frequency of the complex baseband signal down-sampled to the desired signal band such that the center frequency changes from ±fAD/2(n+2) [Hz] to zero.
The reception-waveform-shaping filter unit performs waveform shaping of a reception signal while extracting a main signal of the complex baseband signal, the center frequency of which is frequency-converted into zero. According to this processing, a harmonic (alias) component (the triangular portion) remaining without being able to be removed by the reception half-band filter can be removed.
The digital signal multiplexing apparatus described in Patent Document 1 is explained below. Like the reception half-band filter, the transmission half-band filter of this digital multiplexing apparatus has frequency characteristics corresponding to any one of the bands. Like the reception half-band filter, the transmission half-band filter realizes these frequency characteristics by selecting any complex coefficient corresponding to each of the bands and setting the coefficient in the band-pass filter.
If it is assumed that a sampling frequency of a transmission signal modulated by the modulating unit and shaped by the transmission-waveform-shaping filter is 0.5 fsmap [Hz], the frequency offset unit frequency-offsets a center frequency of a baseband complex signal output from the transmission-waveform-shaping filter by ±0.5 fsmap/4p [Hz]. An offset value is different depending on a band to which filter characteristics of a band-pass filter correspond.
The up-sampler of the transmission half-band filter up-converts a sampling rate to a double with respect to data after frequency conversion. The band-pass filter carries out filtering.
Thereafter, the transmission half-band filter in the past repeats the processing of the up-sample and the processing by the band-pass filter while combining output signals of the transmission half-band filters in the adder until a frequency reaches a desired sampling frequency.